1. Field of the Invention
The present invention relates to an ultrasonic bloodstream imaging apparatus wherein color bloodstream data is superimposed on a monochrome tomographic image.
2. Description of the Related Art
Conventionally, there exists an ultrasonic bloodstream imaging apparatus in which an ultrasonic Doppler method and a pulse reflection method are used in combination to obtain bloodstream data and tomographic image (B-mode image) data with use of a single ultrasonic probe, and the bloodstream data is superimposed on the tomographic image and displayed with color in real time. With this conventional apparatus, the a bloodstream velocity may be measured in the following manner.
An ultrasonic pulse beam is caused to propagate through the blood flowing in a subject or a living body. The ultrasonic beam is scattered by moving blood cells and the center frequency fc of the ultrasonic beam deviates by a frequency component fd due to the Doppler effect. Thus, the received frequency f becomes f=fc+fd. In this case, frequencies fc and fd are given by the following formula: ##EQU1## where v: bloodstream velocity
.theta.: angle between the ultrasonic beam and blood vessel PA1 c: sonic velocity
Thus, by detecting the frequency component fd by which the ultrasonic beam deviates due to the Doppler effect, the bloodstream velocity v can be obtained.
Two-dimensional image display of the obtained bloodstream velocity v is performed in the following manner. As shown in FIG. 1, an ultrasonic pulse beam is sequentially propagated from an ultrasonic probe 1 to a subject in directions a, b, c. . . , thereby to perform sector (or linear) scanning. In this case, the ultrasonic pulse scan control is performed by an ultrasonic bloodstream imaging apparatus as shown in FIG. 2.
At first, the ultrasonic pulse beam is emitted in the direction a several times. The beam is scattered by blood cells in the subject and deviates due to the Doppler effect, and the resulting reflected echo signal is received by the same probe 1 and converted to an electric signal. The electric signal is sent to a receiving circuit 2.
Then, a phase detection circuit 3 detects a Doppler deviation signal. The Doppler deviation signal is sampled, for example, at 256 sampling points provided in the direction of the ultrasonic pulse beam. The Doppler deviation signal sampled at each sampling point is analyzed by a frequency analyzer 4 and delivered to a D.S.C. (digital scan converter) 5. In the D.S.C, the signal is scan-converted. The scan-converted signal is supplied to a display 6, and a bloodstream velocity image in the direction a is displayed as a two-dimensional image in real time.
With respect to the directions b, c,. . . , the same operation is repeated, and bloodstream velocity images (flow velocity distribution image) corresponding to the respective scanning directions are displayed. The flow velocity distribution image is color-displayed and a tomographic image is monochrome-displayed (B/W display), in a composite display manner.
In the conventional apparatus, since the monochrome (luminance)-display B- or M-mode image is simply superimposed on the color (luminance)-display bloodstream velocity data image, the hue of a superimposed area at which the echo level is low and the bloodstream velocity is low is disturbed, resulting in an unnatural image. Thus, exact diagnosis cannot be carried out. This is a main drawback of the conventional apparatus.
In addition, the conventional apparatus has the following drawback. When a color bloodstream data image of a heart is obtained, the wall of the heart rapidly moves like the bloodstream and is also displayed with color, as if blood flowed in the wall.